wp-content/plugins/bbpress/templates/default/bbpress/form-topic.php line 203

and

wp-content/plugins/bbpress/templates/default/bbpress/form-reply.php line 135

You do this by creating a directory under your theme called “bbpress” and copying these two files to there. Then modify the lines as you want, you’ll need to change the word ‘Submit’ in the following towards the end of the line to whatever you want.

<?php _e( ‘Submit’, ‘bbpress’ ); ?>

bbPress will then use these files instead

You could use a translate function, but Submit is such a widely used it might affect any submit on your site